David Cameron's election strategist has denied having any "conversation" with the prime minister over plain packaging on tobacco sold in England and Wales.
Earlier this month, Mr Cameron decided not to go ahead with plans to remove all branding from cigarette packets.
Labour claims adviser Lynton Crosby's work for tobacco giant Philip Morris created a conflict of interest.
The cabinet secretary has ruled out an inquiry but Labour wants clarification of guidelines about Mr Crosby's role.
Mr Cameron has insisted that Mr Crosby, who works for the Conservative Party rather than the coalition, has no role in deciding government policy.
Supporters of plain packaging say it will reduce smoking take-up rates among young people.
But the decision on whether to adopt such a policy - already in place in Australia - was delayed earlier this month, with ministers saying it was important to gather more evidence on its effectiveness.
Cabinet Secretary Sir Jeremy Heywood has rejected Labour calls for an investigation into the role of Mr Crosby, who runs lobbying firm Crosby Textor.
Sir Jeremy said an inquiry would serve "no purpose" since Mr Crosby had not seen government papers nor attended government meetings and his role advising the Conservatives since November 2012 was not, in itself, in conflict with the ministerial code.
In his letter to Labour leader Ed Miliband, Sir Jeremy referred to "principles of engagement" which he said had been agreed regarding Mr Crosby's role as a Tory adviser to prevent any conflicts of interest arising.
- Mr Crosby would not use his role with the Conservatives, and access to ministers, to further his own interests or the private interests of others
- Mr Crosby would not use access to ministers or special advisers to influence or lobby for changes in government policy on behalf of his firm's clients
- Mr Crosby would not help - or given the impression of helping - a client by claiming to have privileged access to ministers, special advisers or officials.
But Labour said the document appeared to have been "stitched together at the last minute" and would be pushing for clarification about when it was drawn up and by whom.
The prime minister has been asked a number of times if he had spoken to Mr Crosby about the plain packaging plans. In answer he has repeatedly said he had not been lobbied by Mr Crosby.
The BBC's deputy political editor James Landale said the Conservatives hoped the release of these "ground rules" and Mr Crosby's statement would draw a line under the affair.
In his statement, Mr Crosby said the prime minister had "repeatedly and clearly said that I have never lobbied him on anything, including on the issue of tobacco or plain packaging of cigarettes."
He also went further, denying that they had ever spoken about the issue.
Mr Crosby said: "What the PM said should be enough for any ordinary person, but to avoid any doubt or speculation let me be clear.
At no time have I had any conversation or discussion with or lobbied the prime minister, or indeed the health secretary or the health minister, on plain packaging or tobacco issues.
"Indeed, any claim that I have sought to improperly use my position as part-time campaign adviser to the Conservative Party is simply false."
Downing Street sources said Mr Cameron was "pleased" Mr Crosby had clarified that they had never had a conversation or discussion about tobacco policy or the plain packaging of cigarettes.